Electrophotographic imaging processes and techniques have been extensively described in the prior art. Generally, such processes have in common the steps of employing a photoconductive element which is prepared to respond to image-wise exposure to electromagnetic radiation thereby forming a latent-electrostatic-charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of the image.
One type of photoconductive element particularly useful in electrophotography employs a composition containing a photoconductive material and optionally an electrically insulating, film-forming, resinous binding material. An integrated electrophotographic element incorporating such a composition is generally produced in a multi-layer type of structure by coating a layer of the above described compositions onto a support previously overcoated with a layer of an electrically conducting material. Alternatively, the above-described composition can be coated directly onto a conductive support made of metal or other suitable conductive materials.
Usually, the desired electrophotographic properties are dictated by the end use contemplated for the photoconductive element. In many such applications, it is desirable for the photoconductive element to exhibit high speed, as measured by an electrical speed or characteristic curve, a low residual potential after exposure and resistance to electrical fatigue. Various other applications specifically require that the photoconductive element be capable of accepting a high surface potential with a low dark decay rate.
In many other applications, it is desirable that the photoconductive element be capable of high speeds and relatively high resolution as measured in terms of lines per millimeter. Typical applications where high resolution images are necessary are one to one microfilm reproductions, and the production of microimages from regular sized images. Ideally, a microfilm duplicating system should provide exact micro-duplicates of existing microfilm frames or microimages or normal-sized copy with no loss in resolution from the original.
High speed "heterogeneous" or "aggregate" photoconductive systems have been developed which exhibit many of the desirable qualities mentioned above. These aggregate compositions are the subject matter of Light, U.S. Pat. No. 3,615,414 issued Oct. 25, 1971, Contois et al, U.S. Pat. No. 3,873,311 issued May 25, 1975 and Gramza et al, U.S. Pat. No. 3,732,180 issued May 8, 1973. These heterogeneous or aggregate photoconductive elements comprise photoconductive compositions containing a continuous polymer phase having dispersed therein co-crystalline particles composed of a pyrylium or thiopyrylium salt and a polymer. However, the resolution obtainable with heterogeneous or aggregate photoconductive elements is not as high as the resolution obtainable with some other types of photoconductive elements having much lower speeds such as the elements disclosed in U.S. Pat. No. 3,542,547 and U.S. Pat. No. 3,938,994.
The use of thiopyrylium dye salts in photoconductive compositions is also disclosed in Contois et al, U.S. Pat. No. 3,973,962, issued Aug. 10, 1976 and Van Allan et al, U.S. Pat. No. 3,250,615 issued May 10, 1966. Certain monomethine thiopyrylium dye salts are also disclosed as sensitizers for photoconductive compositions in Reynolds et al, U.S. Pat. No. 3,938,994 issued Feb. 17, 1976.